1. Field of the Invention
The present invention relates to bandpass filters and particularly to a multiple-pass reflection filter which provides spectral bandpass filtering of ultraviolet, visible and infrared radiation with a very high transmission of wavelengths within a selected passband coupled with exceptional off-band rejection.
2. Description of Related Art
Bandpass filters are used to isolate wavelength intervals without the use of dispersive elements. Commercial bandpass filters fabricated of stacked dielectric layers are widely available for filtering applications from the vacuum UV to the far IR. The simplest bandpass filters are nothing more than small Fabry-Perot interferometers. Their light transmission characteristics depend on the nature of the interference of the light caused by multiple reflections within the dielectric stack. Most bandpass filters are made by combining the dielectric stack interference filter with additional blocking filters such as colored glass or metal films.
Filters are available as edge filters (short-wavelength pass or long-wavelength pass) or true bandpass filters (interval isolation with rejection of both longer and shorter wavelengths). These bandpass filters offer considerable flexibility in the specification of the center wavelength and the width of the passband. The performance of interference-type bandpass filters can be improved by fabricating the filter with several repeating units (cavities) of multi-layer dielectric stacks. Although the passband can be narrowed and the off-band rejection improved in this manner, the overall transmission off-band can not be reduced below a value of T.about.10.sup.-4 without seriously degrading the transmission at the center wavelength as well.
The transmission characteristics of interference-type bandpass filters are extremely angle dependent. In order to realize the anticipated transmission characteristics, the filter must be used in an optical system which ensures that only highly collimated light is incident on the filter. If uncollimated light is allowed to illuminate the filter, then light outside the expected passband may be transmitted. Simultaneously, the transmission of light at the passband may decrease dramatically. If the filter was constructed with metallic blocking layers, the passband can be split into two shifted passbands that transmit oppositely polarized light. Often only very small field angles (&lt;5 degrees) are necessary to cause such deleterious effects.
Although interference and selective absorption are the most common techniques used for optical filters, other methods are also used. Polarization interference, scattering, and selective reflection are alternative techniques used for spectral filtering. In particular, reflection filters offer a simple method to obtain high maximum transmission of wavelengths within a selected passband coupled with a large off-band rejection. These filters are simply mirrors having reflectivity in the wavelength range of interest, but which absorb or transmit all other wavelengths. Reflection filters have been used to isolate wavelength intervals from the UV to the IR. Spectral filtering in the UV can be a particularly demanding application for which a reflection filter is well suited. For example, the transmission of a four-pass reflection filter manufactured by Schott Glasswork had a peak transmission of 0.91 at 250 nm and transmission of only 0.001 at 220 nm and 285 nm. The peak of the passband, as well as its width, is largely determined by the characteristics of the reflective coating.